How to dynamically change the selection criterion in the Entity framework?

Question

C++ / C#

Good day, Toasterians!
I decided to get acquainted with the Entity Framework. For the universality of the script, you need to make a selection in the same table according to one of the three criteria. In the future, with all the results will be the same processing.
Current implementation:

var targetType = "G"; //целевой тип отчета. G - группа, T - преподаватель, C - кабинет
            int targetId = 2; // ID целевой группы/препода/кабинета
            var loadDb = db.Events.Where(l => l.cabinet == targetId.ToString());
            switch (targetType)
            {
                case "G": loadDb = db.Events.Where(l => l.group == targetId.ToString()); break;
                case "T": loadDb = db.Events.Where(l => l.teacher == targetId.ToString()); break;
                case "C": loadDb = db.Events.Where(l => l.cabinet == targetId.ToString()); break;
                default: targetType = "Error"; break;
            }

In this case, the variable is initialized by an extra query to the database. How to make a query look like this:

switch (targetType)
            {
                case "G":  запрос  = l => l.group == targetId.ToString(); break;
                case "T":  запрос  = l => l.teacher == targetId.ToString(); break;
                case "C":  запрос  = l => l.cabinet == targetId.ToString(); break;
                default: targetType = "Error"; break;
            }
            if (targetType != "Error") 
            {
            var loadDb = db.Events.Where( запрос );
            }

Answer 1

You are a little confused)
When you execute a line
You don't have a database query. In this line, only the query itself is created, that is, the sql code is generated based on your query. It doesn't go anywhere in the base. And is assigned to the loadDb variable
Next, when you do, for example,
You write another generated request to the loadDb variable. You are simply overwriting the variable. In this case, requests to the database are also not received.
Answering your question, most likely you want to impose an additional condition on one request. In your code it will look something like this:

var loadDb = db.Events.Where(l => l.cabinet == targetId.ToString());
            switch (targetType)
            {
                case "G": loadDb = loadDb.Where(l => l.group == targetId.ToString()); break;
...

And the very execution of a query to the database occurs when you call methods that return a result, for example
var res = loadDb.ToList()

Answer 2

What is the problem? Where performs a predicate, i.e. you need something like

Func<тип записи, bool> запрос;
switch (targetType)
            {
                case "G":  запрос  = l => l.group == targetId.ToString(); break;
                case "T":  запрос  = l => l.teacher == targetId.ToString(); break;
                case "C":  запрос  = l => l.cabinet == targetId.ToString(); break;
                default: targetType = "Error"; break;
            }
            if (targetType != "Error") 
            {
            var loadDb = db.Events.Where( запрос );
            }